Syntheses, Crystal Growths, and Nonlinear Optical Properties for 2

Oct 18, 2007 - State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuz...
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Syntheses, Crystal Growths, and Nonlinear Optical Properties for 2-Carboxylic acid-4-nitropyridine-1-oxide Crystals with Two Different Arrangements of Chromophores Wenshi Wu,†,‡ Dongsheng Wu,† Wendan Cheng,*,† Hao Zhang,† and Jincao Dai‡

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 11 2316–2323

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, People’s Republic of China, and College of Material, Huaqiao UniVersity, Quanzhou, Fujian 362021, People’s Republic of China ReceiVed March 11, 2007; ReVised Manuscript ReceiVed August 19, 2007

ABSTRACT: A novel chromophore 2-carboxylic acid-4-nitropyridine-1-oxide (POA) has been synthesized, and the selective control of its crystal growth leads to two new second-order nonlinear optical (NLO) crystal materials named POA-I and POA-II accordingly. POA-I and POA-II are both orthorhombic, with space group P212121. Their cutoff ultraviolet–visible absorptions are located at about 380 nm. The Kurtz powder measurement shows that their second harmonic generation (SHG) efficiencies are about 4.6 and 9.8 times as large as that of KH2PO4 (KDP), respectively. The calculations of the SHG NLO susceptibilities are performed by the time-dependent density-functional theory (TDDFT) method coupled with the sum-over-state formula. It is found that a larger susceptibility originates from larger charge transfers and a stronger interaction among chromophores in the POA-II crystal.

1. Introduction Some advances in organic nonlinear optical (NLO) materials have led to renewed interest in the investigations of this field during past years.1–3 It is a great challenge to increase the level of control over and to understand the intermolecular interactions and packing forms in organic bulk materials. Design of noncentrosymmetrical crystal is controlled in crystal growth processes. Some successful examples were reported in designing organic solid NLO materials.4–8 In this work, we develop two crystals as candidates of second-order NLO materials and employ a crystal-engineering method9,10 to control the crystal growths. Two strategies are considered during the designs: (1) a chromophore with large hyperpolarizability is designed, and the orientation of the new chromophore of 2-carboxylic acid4-nitropyridine-1-oxide (POA) is optimized; (2) the growing conditions are controlled to obtain two POA crystals with two different crystallizing forms but with the same space group. The design clue of the POA noncentrosymmetrical crystal is inspirited by the work of Zyss and Chemla.6 In the tentative control of the noncentrosymmetry in the crystalline solid, they employed the vanishing dipole moment strategy to demonstrate the NLO crystal of 3-methyl-4-nitropyridine-1-oxide (POM). It indicated that the substituted 4-nitropyridine-1-oxide in the 3 position by methyl, bromine, and chlorine has second harmonic generation (SHG) effects; however, it has no SHG effect from the substituent of carboxylic acid in the 3 position.6 In fact, the POA is volatile in the room temperature based on our experiment, and it is unstable because of the steric interaction between nitro and carboxylic groups in 3-carboxylic acid-4-nitropyridine-1-oxide based on our theoretical calculation. It is well-known that COOH is a planar configuration and is made up of a hydroxyl group (–OH) bonded to a carbonyl group (–CdO). The COOH function results from the competition interaction between the –OH and –CdO functions within the COOH. If the 4-nitropyridine-1-oxide in the 2 position is substituted by COOH, the molecular coplane will be formed by the COOH group with 4-nitropyridine-1-oxide and the steric * To whom correspondence should be addressed. E-mail: [email protected]. † Chinese Academy of Sciences. ‡ Huaqiao University.

Scheme 1. Preparation of POAa

a (i) Pyridine-2-carboxylic acid, glacial acetic acid, 30% hydrogen peroxide, 80 °C oil bath for 24 h. (ii) Sulfuric acid (specific gravity of 1.84), fuming yellow nitric acid, 100–105 °C oil bath for 4 h.

interaction will be avoided between COOH and NO2. Furthermore, it is possible that the molecular conjugation will be enhanced and crystal centrosymmetry will be removed, adding the COOH in the 2 position of 4-nitropyridine-1-oxide. Following this clue, we take the pyridine-2-carboxylic acid as the original material, and the synthetic route is outlined in Scheme 1. Finally, we perform the measurements of electronic absorption spectra and SHG of POA powder and the calculations of frequency-doubling coefficients of POA crystals.

2. Experimental and Computational Procedures Syntheses of POA. A mixture of glacial acetic acid and pyridine2-carboxylic acid contained in a round-bottomed flask was shaken, with cold (0–5 °C) 30% hydrogen peroxide added, and the mixture was heated in an oil bath for 24 h. After the excess acetic acid and water were removed under reduced pressure, the residue was cooled to 0–5 °C in an ice–salt bath and cold (0–5 °C) 40% aqueous sodium hydroxide solution was added slowly with shaking and neutralized with shaking. The solution was extracted with chloroform, and the extracts were filtered and concentrated by distillation under reduced pressure. The residue was cooled, and the white crystalline powder of 2-carboxylic acid-pyridine-1-oxide was obtained. This white powder has a melting point of 172 ( 0.5 °C (ca. 78%). The white powder of this middle product was added to cold (0–5 °C) sulfuric acid (specific gravity of 1.84) contained in a round-bottomed flask, immersed in an ice–salt bath. The resulting mixture was cooled to about 10 °C, and fuming yellow nitric acid was added with shaking. An efficient spiral condenser was attached to the flask and latter was placed in an oil bath. The temperature was slowly raised to 95–100 °C, and the heating was continued at 100–105 °C for 4 h. The reaction mixture was cooled to

10.1021/cg0702378 CCC: $37.00  2007 American Chemical Society Published on Web 10/18/2007

2-Carboxylic acid-4-nitropyridine-1-oxide (POA) 10 °C and poured onto crushed ice contained in a beaker. The addition of sodium carbonate monohydrate in small portions with stirring caused the separation of the yellow crystalline product along with sodium sulfate. The mixture was then allowed to stand for 3 h to expel nitrogen oxides. The yellow solid was collected by suction filtration, thoroughly washed with water, and rendered as dry as possible on the filter. The filtrates were transferred to a separator funnel. The collected solid was extracted with portions of boiling chloroform; the combined extracts were used to extract the aqueous filtrates contained in the separator funnel; and the extraction was repeated with several fresh portions of chloroform. The combined chloroform extracts were given preliminary drying over anhydrous sodium sulfate and evaporated to dryness by distillation under reduced pressure. The pale yellow-colored product of POA was filtered by suction; the filtrates were removed and saved; and the collected solid was washed with ether and dried. The pale yellow powder of POA has a melting point of 146 ( 1 °C (ca. 69%). Crystal Growths. The pale yellow powder of POA was dissolved in a boiling mixture of methanol and dichloromethane. The solution was cooled to about 30 °C and carefully filtered. The filtered solution was maintained a few degrees above the saturation temperature for homogenization, and the microcrystals were formed in the mother liquid. It was placed in the constant temperature bath, whose temperature was controlled to the accuracy of 35 ( 0.05 °C, to ensure that the microcrystals were dissolved sufficiently. Growth commenced at this temperature. Solvent evaporation was controlled by tight covering at the top of the beakers using polyethene sheets. Transparent pale yellowcolored crystal (POA-I crystal) was harvested after a period of 20 days, with a melting point of 148 ( 0.5 °C. The grown crystal was prism in shape and had the dimensions of 4.0 × 3.0 × 3.0 mm3. A mixture of dichloromethane and pale yellow powder of POA was added to a mixture of methanol and potassium hydroxide. After 48 h, a mixture of water and glacial acetic acid that serves as a pH control was added to the above-mentioned mixture and carefully filtered. Transparent pale yellow-colored crystal (POA-II crystal) was harvested after a period of 7 days, with a melting point of 156 ( 0.5 °C. The grown crystal was needle in shape and had the dimensions of 0.80 × 0.18 × 0.14 mm3. Crystal Structure Determinations. Single-crystal X-ray diffraction were carried out to investigate the lattice parameters for POA crystals using a Siemens Smart CCD diffractometer with Mo KR radiation (λ ) 0.71073 Å) at room temperature. The structures were solved by direct methods using SHELXS-9711 and refined by full-matrix least-squares methods using SHELXL-97. Anisotropic displacement parameters were refined for all nonhydrogen atoms. The hydrogen atoms connecting to oxygen were located in a difference Fourier map, and others were added in the riding model. Isotropic displacement parameters were refined for hydrogen atoms. The final cycle of refinement gave R ) 0.0301 and wR ) 0.0827 for the POA-I crystal and R ) 0.0482 and wR ) 0.1131 for the POA-II crystal. The weight was defined as w ) 1/[σ2(Fo2) + 0.0441(P)2 + 0.2184P] for the POA-I crystal and w ) 1/[σ2(Fo2) + 0.0584(P)2 + 0.4301P] for the POA-II crystal, where P ) (Fo2 + 2Fc2)/3 [Cambridge Crystallographic Data Center (CCDC) 618073 and 618074]. The crystallographic data are summarized in Table 1. Spectra Measurements. The absorption spectrum was recorded by a UV-3101PC spectrometer at 300 K from 200 to 800 nm, and the NLO property of the POA powder was measured by the Kurtz powder SHG method. A Q-switched mode-locked Nd-YAG laser (λ ) 1064 nm) was used as the illuminating source, and potassium dihydrogen phosphate (KDP) was used as the reference material. Computational Descriptions. Density functional theory (DFT) calculations were performed with the GAUSSIAN 03 package.12 The hybrid B3LYP13 functional with 6-311+G(3df,2p) basis set was applied to evaluate the interaction energy among molecules within one unit cell of POA crystals. The properties of excited states were calculated by using the time-dependent density functional theory (TDDFT) with BP8614,15 functional and 3-21G* basis set. With the calculated dipole moments and transition moments of excited states of a 4-molecule cluster in a POA crystal, the sum-over-state (SOS) formalism16,17 was employed to evaluate the NLO responses of the 4-molecular cluster and the optical physical property superposition of the 4-molecular cluster is applied to obtain optical susceptibility of the POA crystal.

Crystal Growth & Design, Vol. 7, No. 11, 2007 2317 Table 1. Crystallographic Data and Refinement Details of the X-ray Analysis of POA crystal I

crystal II

formula formula weight crystal system space group a (Å) b (Å) c (Å) V (Å3) Z Dcalcd (mg m-3) µ (mm-1) F(000) crystal size (mm) θmin, θmax (deg) index range h k l number of independent reflections Rint number of observed reflections corrections R wR GOF extinction coefficient largest difference peaka

C6H4N2O5 184.11 orthorhombic P212121 13.2440(7) 5.9984(2) 9.0482(3) 718.81(5) 4 1.701 0.152 376 0.70 × 0.40 × 0.30 2.73, 25.02

C6H4N2O5 184.11 orthorhombic P212121 5.5509(3) 9.8139(3) 13.2554(7) 722.10(7) 4 1.694 0.151 376 0.40 × 0.18 × 0.14 2.58, 25.00

-5 f 15 -7 f 6 -10 f 10 1228

-6 f 6 -11 f 5 -12 f 15 1079

0.0168 1171

0.0301 959

Lp 0.0301 0.0827 1.020 0.048(6) 0.143 (-0.107)

Lp 0.0482 0.1131 0.948 0.019(5) 0.142 (-0.176)

(∆/σ)mean,max

0.000, 0.000

0.000, 0.000

(hole) (e– Å-3)

a

Largest peak (hole) in the difference Fourier map.

Figure 1. ORTEP diagram of POA.

3. Results and Discussion Crystal Structures of POA. The molecular configuration parenthesis atomic symbol of POA is presented in Figure 1, and selected interatomic distances and angles are presented in Table 2. Single-crystal X-ray analysis showed that the atoms of crystal POA-I are all in a plane. Its least-square equation is 6.389(5)x + 2.944(1)y - 6.565(3)z - 2.064(4) Å ) 0, with 0.0704 Å being the root-mean-square deviation (rmsd). The length of the O(1)–N(1) bond [1.323(2) Å] is longer than the corresponding bond [1.285(3) Å] in 2-POM18 and the bond [1.288(2) Å] in POM.19 The length of the C(3)–N(2) bond [1.469(3) Å] is longer than the corresponding bond [1.450(3) Å] in 2-POM and is in agreement with the corresponding bond [1.460(3) Å] in POM. The lengths of O(4)–N(2) and O(5)–N(2)

2318 Crystal Growth & Design, Vol. 7, No. 11, 2007 Table 2. Selected Bond Lengths (Å) and Angles (deg) of the Title Compounds bond

crystal I (Å)

crystal II (Å)

C(1)–N(1) C(1)–C(2) C(1)–C(6) C(2)–C(3) C(2)–H(2A) C(3)–C(4) C(3)–N(2) C(4)–C(5) C(4)–H(4A) C(5)–N(1) C(5)–H(5A) C(6)–O(3) C(6)–O(2) N(1)–O(1) N(2)–O(4) N(2)–O(5) O(2)–H(02)

1.368(2) 1.377(3) 1.511(3) 1.369(3) 0.9300 1.376(3) 1.469(3) 1.367(3) 0.9300 1.353(2) 0.9300 1.203(3) 1.305(3) 1.323(2) 1.218(3) 1.221(3) 1.0701

1.363(4) 1.360(4) 1.505(4) 1.371(4) 0.9300 1.371(5) 1.472(4) 1.361(5) 0.9300 1.352(4) 0.9300 1.206(4) 1.306(4) 1.318(4) 1.216(4) 1.220(4) 0.8925

angle

crystal I (deg)

crystal II (deg)

N(1)–C(1)–C(2) N(1)–C(1)–C(6) C(2)–C(1)–C(6) C(3)–C(2)–C(1) C(2)–C(3)–C(4) C(2)–C(3)–N(2) C(4)–C(3)–N(2) C(5)–C(4)–C(3) N(1)–C(5)–C(4) O(3)–C(6)–O(2) O(3)–C(6)–C(1) O(2)–C(6)–C(1) O(1)–N(1)–C(5) O(1)–N(1)–C(1) C(5)–N(1)–C(1) O(4)–N(2)–O(5) O(4)–N(2)–C(3) O(5)–N(2)–C(3)

118.9(2) 120.7(2) 120.3(2) 119.2(2) 121.6(2) 119.2(2) 119.1(2) 118.2(2) 120.7(2) 123.9(2) 119.5(2) 116.6(2) 118.0(2) 120.7(2) 121.4(2) 124.2(2) 118.0(2) 117.8(2)

118.9(3) 120.4(3) 120.6(3) 120.3(3) 120.5(3) 119.6(3) 119.9(3) 118.4(3) 121.1(3) 123.0(3) 119.7(3) 117.3(3) 118.3(3) 120.9(3) 120.8(3) 124.4(4) 117.8(3) 117.7(4)

bonds [1.218(3) and 1.221(3) Å] are in agreement with the corresponding bonds [1.223(3) and 1.238(3) Å] in 2-POM and bonds [1.224(3) and 1.226(3) Å] in POM. The environments of oxygen atoms in carbonic acid are not the same. One oxygen [O(2)] is linked by a hydrogen [the length of O(2)–H(02) is 1.0701 Å], and another oxygen [O(3)] is not. The length of the O(3)–C(6) bond [1.203(3) Å] is shorter than that of O(2)–C(6) [1.305(3) Å]. There are intramolecular hydrogen bonds in the

Wu et al.

crystal POA-I. The hydrogen bond length of O(2)–H · · · O(1) is 2.460(2) Å. The angle of the O(2)–H · · · O(1) hydrogen bond is 159.6°. There are four molecules in one unit cell. The molecules are not parallel with each other in the unit cell. The dihedral angles between neighbored molecules are 88.46(3)°, 59.13(3)°, and 59.08(3)°, respectively (shown in Figure 2a). It can be seen from the one-dimensional chain structure that there exist C(5)–H · · · O(3a) (symmetry code: a, -1/2 + x, 1/2 y, 1 - z) intermolecular hydrogen bonds in the crystal POA-I (shown in Figure 3a). The C(5)–H groups act as a proton donor in the hydrogen-bonded system.20 The bond lengths of C(5)–H and H · · · O(3a) are 0.930 and 2.576 Å, respectively, and the bond angle of C(5)–H · · · O(3a) is 140.1°. The calculation using B3LYP in conjunction with the 6-311+G(3df,2p) basis set indicates that the binding energy of the C(5)–H · · · O(3a) hydrogen bond is about 0.64 kJ mol-1, which is less than 1 kJ mol-1 and expected to be resonable.14 The atoms of crystal POA-II are all in a plane too. Its leastsquare equation is 3.015(3)x - 8.226(4)y + 0.633(8)z 4.285(5) Å ) 0, with 0.0332 Å being the rmsd, excelled deviation of crystal POA-I. The lengths of the O(1)–N(1) bond [1.318(4) Å], C(3)–N(2) bond [1.472(4) Å], O(4)–N(2) bond [1.216(4) Å], O(5)–N(2) bond [1.220(4) Å], O(3)–C(6) bond [1.206(4) Å], and O(2)–C(6) bond [1.306(4) Å] are in agreement with those of crystal POA-I, respectively. The environments of oxygen atoms in carbonic acid are not the same too. The length of the O(2)–H(02) bond (0.8925 Å) is shorter than that (1.0701 Å) of crystal POA-I. There are intramolecular hydrogen bonds in the crystal POA-II too. The hydrogen bond length of O(2)–H · · · O(1) is 2.461(2) Å. The angle of the O(2)–H · · · O(1) hydrogen bond is 151.5°. The stacking of crystal POA-II is not the same as that of crystal POA-I. The dihedral angles between neighbor molecules in crystal POA-II are 66.09(4)°, 65.81(4)°, and 5.47(8)°, respectively (shown in Figure 2b). It can be seen from the one-dimensional chain structure that there exist C(4)–H · · · O(5a) (symmetry code: a, -1/2 + x, 1 /2 - y, 1 - z) intermolecular hydrogen bonds in the crystal POA-II (shown in Figure 3b). The bond lengths of C(4)–H and H · · · O(5a) are 0.930 and 2.381 Å, respectively, and the bond angle of C(4)–H · · · O(5a) is 154.0°. The two molecules in the POA crystals are held together by a weak hydrogen bond of C–H · · · O, and one of the molecules donates one hydrogen bond to the other. The one-dimensional chains are formed, and the

Figure 2. Packing arrangement in a unit cell for (a) POA-I and (b) POA-II.

2-Carboxylic acid-4-nitropyridine-1-oxide (POA)

Crystal Growth & Design, Vol. 7, No. 11, 2007 2319

Figure 3. Packing arrangement with hydrogen bonds for (a) POA-I and (b) POA-II.

Figure 4. Electronic absorption spectra of POA powder: (a) POA-I and (b) POA-II.

POA crystals are stabilized by these weak hydrogen bonds. In effect, weak hydrogen bonds that are employed collectively can work as effectively as their stronger counterparts.21 Electronic Spectra and Electronic Structures. Figure 4 shows the absorption spectra of POA-I and POA-II powder determined from reflection measurements. The strongest absorp-

tion peaks are located at 274 and 278 nm, and the second ones are localized at 373 and 353 nm for the POA-I and POA-II powder, respectively. The calculated absorption spectra of 1 molecule of POA at the TDBP86/3-21G* level are plotted in Figure 5. The measurement absorption bands from 180 to 340 nm and from 340 to 440 nm correspond to calculated ones from

2320 Crystal Growth & Design, Vol. 7, No. 11, 2007

Wu et al.

Figure 5. Calculated absorption spectra of 1 molecule: (a) POA-I and (b) POA-II.

180 to 310 nm and from 310 to 350 nm for POA-I and correspond to calculated ones from 180 to 300 nm and from 300 to 380 nm for POA-II, individually. Here, we noted that the absorption spectrum of the crystal is different from that of a single molecule, and intermolecular excitonic interactions result in the red shift of absorption spectra from the molecule to the crystal. These results were discussed in detail in the published paper by Pierre et al.22 We make systemic comparisons between the single molecule and corresponding crystal to give an assignment of the crystal absorption spectrum. Our calculations indicate that the maximum absorption band observed in the experiment corresponds to the S11 state that has a larger oscillator strength. The analysis of the TDDFT wave function indicates that the S11 state has π–π* transition characteristics. This shows that the largest absorption band of POA crystals mainly originates from π–π charge-transfer transitions. SHG Susceptibilities. To include the supramolecular interactions in the POA-I and POA-II crystal materials, we choose the 4-molecular cluster from one unit cell as the basic model to calculate the second-order polarizabilities β(-2ω;ω,ω) of the SHG process. The x, y, and z directions in the Cartesian coordination correspond to the unit cell a, b, and c axes of the orthorhombic system, respectively. For the calculations of β, we first consider how to truncate the infinite SOS expansion to a finite one. Figure 6 shows the plots of the calculated secondorder polarizabilities βxyz, βyzx, βzxy, and βtot(-2ω;ω,ω) at the TDBP86/3-21G* level versus the number of states for POA-I and POA-II at an input photon energy of hω ) 1.165 eV. The curve of βtot(-2ω;ω,ω) of POA-I shows good convergence after state 32, providing confidence that the truncation with 120 states is reasonable. The 13th state makes a large contribution to the βtot value, and the calculated βtot value including the first 13 states is about 63% of the βtot value including 120 states. While the βtot(-2ω;ω,ω) of POA-II is considered, the curve has oscillation and the convergence is not as good as that of POAI. However, it can be found that all of the values after 6 states fall in the range of 9–15 × 10-30 cm5 esu-1, which is acceptable for the deviations. The 4th state of the 4-molecule cluster in the POA-II crystal makes the largest contribution to the βtot value. Both the 13th state of POA-I and 4th state of POA-II have the significant configurations of HOMO-3 f LUMO, where HOMO-3 is the π-bonding orbital mainly located at one monomer but with a small mixing from the neighboring monomer and LUMO is the π*-antibonding orbital located on another monomer in the 4-molecule cluster (shown in Figure 7). From the analysis of the molecular orbitals and configuration contributions, it can be found that charge transfers between the monomers mainly make contributions to the hyperpolarizability of POA-I and POA-II 4-molecule clusters. Furthermore, struc-

Figure 6. Convergent behavior of β values with the number of states for the 4-molecule cluster: (a) POA-I and (b) POA-II.

ture–property relations at the molecular level are discussed in detail for POA molecules. The molecules in both the POA-I and POA-II crystals are planar, with 0.0704 and 0.0332 Å being the rmsd of atoms, and intramolecular hydrogen bonds form between the hydroxyl group of COOH and oxygen of N-oxide for the molecules as mentioned above. The COOH is a planar structure and is composed of hydroxyl and carbonyl groups. The changes in chemical and physical properties result from the competition interaction between the hydroxyl and carbonyl functions within the COOH. A hydrogen bond formed with the neighbor oxygen of N-oxide donates a proton and results in a reduced ability of the charge transfer of N-oxide; however, the excess charges of the carbonyl group withdrawing from the oxygen atom of the hydroxyl group in the COOH group transfer into 4-nitropyridine-1-oxide while the hydrogen bond forms. The accepting charge effect of the nitro group has not substantial changes because of the compensatory effect between the hydroxyl and carbonyl groups in the POA molecules. Hence, the POA chromophore would not interfere destructively with the basic charge-transfer mechanism from the N-oxide group to the nitro group, which results in the expected optical nonlinearity. The first row of Figure 6 gives the plots of HOMO and LUMO of the POA single molecule, and they show the

2-Carboxylic acid-4-nitropyridine-1-oxide (POA)

Crystal Growth & Design, Vol. 7, No. 11, 2007 2321

Figure 7. Frontier molecular orbitals.

evidence that the charge transfers from the N-oxide and COOH groups to the nitro group while an electron is promoted from HOMO to LUMO. In contrast, the atoms in POM are not coplanar because the methyl group in the 3 position of POM is not a planar structure itself. Accordingly, the methyl induces a rotation of the nitro group and results in a reduced conjugation between the donor oxygen of the oxide and the nitro acceptor. This means that a reduced charge transfer from the N-oxide to the nitro groups will result in a weak optical response of the POM chromophore. Additionally, the intermolecular hydrogen bonds formed from the C–H of the pyridine ring and O of CdO in the COOH group also lead to a quasi-optimal molecular orientation and enhancement of the NLO response in the POA crystals. These conclusions have also been found in N-(4nitrophenyl)-L-prollnol (NPP) and 2-methyl-4-nitroaniline (MNA) crystals.23 In fact, the molecule or molecular cluster is a building component of bulk materials; the physical properties of the bulk materials are dependent upon their molecular components, the manners of arrangement, and the linking of the components in the crystalline state. The molecular clusters are taken as calculating models in this study, which are exactly 4 molecules in a unit cell of POA-I and POA-II, respectively. The inherent noncovalent force interaction and optimal arrangement among the 4-molecular cluster are included in the extended solid-state architectures. For the solid state constructed through the

noncovalent force interaction among the molecules, the optical properties of the solid state simply arise from the 4-molecular structure and the packing manner among the molecules. For this reason, the studies of optical properties of materials predigest the investigations of molecular cluster optical properties. Accordingly, the NLO susceptibility χ(2) abc of the bulk material can be estimated from the first-order hyperpolarizability of the 4-molecular cluster βabc, corrected by the local field factor along the three crystal axes. 2 χabc (-2ω;ω,ω) ) NLβabc(-2ω;ω,ω) ( )

Here, N can be evaluated with N ) 1/Vcell (Vcell is the volume of the unit cell of the POA crystal). The local field correction factor is calculated from L ) fa 2ωfb ωfc ω, and the fω ) [nω2 + 2]/3 ) 1/[1 – (4π/3)NRω], with the assumption of the Lorentz–Lorenz local field,24 in which nω is the refractive index and Rω is the first-order microscopic polarizability. With SOS formalism, we can evaluate Rω and thus calculate the macroscopic susceptibility. The calculated main values of χ(2) are listed in Table 3 for the SHG optical process at an input wavelength of 1064 nm. The Kurtz powder SHG measurement shows that POA-I and POA-II exhibit powder SHG efficiency about 4.6 and 9.8 times as large as that of KDP. The selected value of the KDP d36 coefficient originating from a averaging data is 1.6 × 10-9 esu,25 and thus, our measured d coefficients (2d ) χ(2)) are about 7.36 × 10-9 (14.72 × 10-9) esu and 15.68 ×

2322 Crystal Growth & Design, Vol. 7, No. 11, 2007 Table 3. Parameters for Calculating the NLO Susceptibilities of POA at 1064 nm for the SHG Optical Process

POA-I POA-II a

Na

(2) b χ123

(2) b χ231

(2) b χ312

(2)b χtot

χ(2) (experiment)b

1.390 1.385

0.78 6.69

1.02 7.76

2.46 6.95

11.56 21.80

14.72 31.80

At 1021 cm-3. b At 10-9 esu.

10-9 (31.36 × 10-9) esu, respectively. Accordingly, the (2) estimated values of χtot listed in Table 3 are smaller than those in the experiment but are quite a satisfactory estimation. In our cluster model, molecular orientations and interactions in the crystalline state have been considered in 4-molecular cluster calculations. The influence of individual molecular orientations in the crystalline state on the optical property has not been analyzed in details. At the moment, we make some suggestions about structure–property relations in terms of the definition of the “b” tensor, which is a dimensionless parameter developed by Zyss and Oudar.26 The crystal NLO parameter of d varies directly with angular factors of molecular orientation within the crystalline state, while the b tensor is taken as a unit. For the 222 point group, the optimal value of the angular factor (sin φ cos φ con θ sin2 θ; φ ) 45° and θ ) 54.7°) is 0.1924 for maximizing crystal nonlinearity in regular tetrahedral configuration constructed by the 4 dipole axes of 4-molecular plans.26 Both the POA-I and POA-II crystals belong to the 222 point group, and the 4 dipole axes of 4-molecular plans form a pseudotetrahedral structure; the angular factor is 0.1622 (about φ ) 29.5° and θ ) 59°) and 0.1550 (about φ ) 33° and θ ) 66°), separately for POA-I and POA-II crystals. It is shown that the angular factor of the POA-II crystal has a larger difference from the ideal optimum value. However, the POA-II crystals have larger NLO parameters compared to the ones of the POA-I crystals in both our calculations and measurements. These results indicate that the one-dimensional dipole model is not a good treatment on the POA crystals. It is a good scheme that the POA crystals are treated to employ the multipolar (octupolar) molecular engineering method27,28 because of the COOH substituent in the 2 position of 4-nitro-pyridine-1-oxide. Distinguishing between the POA-I and POA-II Crystals. First, the preparing measure and conditions are different between the POA-I and POA-II crystals. The POA-I crystal is formed through the filtrating and recrystal processes; however, the POA-II crystal is formed only through the filtrating process. Second, the molecular arrangements are different between the POA-I and POA-II crystals. The dihedral angles between neighbor molecules in a unit cell are 88.46°, 59.08°, 59.13°, 59.13°, 59.08°, and 88.46°, respectively, for POA-I crystals and are 66.09°, 5.47°, 65.81°, 65.81°, 5.47°, and 66.09°, respectively, for POA-II crystals. Third, the physical properties between the POA-I and POA-II crystals are different. The melting point and SHG effect are larger for POA-II crystals than those of POA-I crystals. A high melting point of the POA-II crystal arises from a strong interaction between molecules in the crystal. The calculated intermolecular interaction energies of the 4-molecule cluster in a unit cell are -33.9637 and -55.1982 kJ/mol for POA-I and POA-II crystals, respectively. A large SHG response of the POA-II crystal comes from a large hyperpolarizability of the 4-molecule cluster of the POA-II crystal. The large charge transfers between the molecules will make a large contribution to the hyperpolarizability of the 4-molecular cluster of the POAII crystal. The calculated important state contributing mostly to hyperpolarizabilities has a small transition moment and large electronic transition energy of the POA-I molecular cluster (0.2801 Debye and 2.3898 eV from the ground state to state 13) and has a large transition moment and small transition

Wu et al.

energy of the POA-II molecular cluster (0.6865 Debye and 2.0435 eV from the ground state to excitation state 4).

4. Conclusions A novel chromophore and new NLO materials of POA have been synthesized, and their structures have been determined by the single-crystal X-ray diffraction technique. Under controlling conditions of crystal growth, we have obtained the POA crystals of two crystallizing forms. They have different arrangements of molecules in one unit cell. The stabilization of the POA-II crystal is larger than that of the POA-I crystal, in view of the melting point and calculated unit-cell energies or interaction energies between the molecules. The calculated and measured electronic absorption spectrum of the POA-II crystal or powder is red-shift as compared to that of the POA-I crystal, and the experimental and theoretical SHG is larger for POA-II than for POA-I. The obtained results show that the molecular arrangements and interactions among the molecules in the crystals are essential in the design of NLO materials. Acknowledgment. The authors are grateful to the National Science Foundation of China (20373073 and 90201015), the National Basic Research Program of China (2004CB720605), the Fund of Fujian Key Laboratory of Nanomaterials (2006L2005), and the National Science Foundation of Fujian Province of China (2003F006 and 2006J0164) for financial support. Supporting Information Available: Details of crystallographic data for POA-I and POA-II (in CIF format). This material is available free of charge via the Internet at http://pubs.acs.org.

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